1. Field of the Invention
The invention relates to crosslinkable compositions of silane-crosslinking prepolymers, to processes for producing them, and to the use thereof as adhesives and sealants, especially for the bonding of substrates.
2. Description of the Related Art
Polymer systems which possess reactive alkoxy silyl groups have a long history. On contact with water or atmospheric moisture, these alkoxy silane-terminated polymers are capable even at room temperature of undergoing condensation with one another, accompanied by elimination of the alkoxy groups. One of the most important applications of such materials is the production of adhesives.
Adhesives based on alkoxy silane-crosslinking polymers, then, in the fully cured state exhibit not only good qualities of adhesion to a number of substrates, but also very good mechanical properties, being capable of being not only highly elastic but also of possessing tensile strength. Another critical advantage of silane-crosslinking systems relative to numerous other adhesive and sealant technologies (to isocyanate-crosslinking systems, for example) is the non-objectionable toxicological properties of the prepolymers.
Numerous applications prefer one-component systems (1K systems) which cure on contact with atmospheric moisture. The key advantages of one-component systems are above all their very great ease of application, since in this case there is no need for the user to mix different adhesive components. In addition to the saving in time/labor and the reliable avoidance of possible metering errors, there is also no need, with one-component systems, to process the adhesive/sealant within a usually fairly narrow time window, as is the case with multicomponent systems after the two components have been thoroughly mixed.
One particular variant of adhesives based on alkoxy silane-crosslinking polymers is described in DE-A 10 2011 006130 and also DE-A 10 2011 081264, comprising phenyl silicone resins as well as the silane-crosslinking polymers. The corresponding resin additives not only improve the adhesion on numerous different substrates, but also lead—particularly with the high resin contents described in DE-A 10 2011 081264—to adhesives, which following their complete curing, exhibit considerably improved hardness and tensile shear strength.
Nevertheless, the systems actually described in the prior art are confined to materials which comprise polymers having what are called alpha-alkoxy silane groups, where the alkoxy silyl group is separated only by one CH2 group from a heteroatom. This particular type of polymer is distinguished by particularly high reactivity, and so the use of high-activity catalysts, such as organotin compounds, amidine or guanidine derivatives, for example, is unnecessary. This is very advantageous insofar as high-activity catalysts may catalyze exchange reactions in which Si—O—Si bonds of the added phenyl silicone resins react with the Si—OCH3 groups of the alkoxy silane-functional polymers. Exchange reactions of this kind will form Si—O—Si bonds between the alkoxy silane-functional polymers, possibly leading to a drastic increase in viscosity or to complete gelling of the material. The corresponding formulation would no longer be stable on storage.
A further disadvantage of these systems described in the prior art, however, is that almost no commercially available types of polymers contain the required high-reactivity alpha-alkoxy silane groups, instead possessing what are called gamma-alkoxy silane groups, in which the alkoxy silane unit is connected by a (CH2)3 spacer to the adjacent heteroatom. This means a massive restriction on the raw materials which can be used, particularly since the few alpha-alkoxy silane-functional polymers that are available commercially also have systemic disadvantages such as, for example, comparatively high costs, as a consequence of their production, or else comparatively poor resilience. There are many profiles of properties that cannot be produced with the existing technology, therefore.